How are the masses of atoms measured?

Mass Spectrometer


A mass spectrometer is a device which is used to measure the relative numbers of molecules and atoms of different masses. The mass spectrometer is a physical way of analysing elements and the mass of the particles that create them. Previously chemical methods were used but these proved less accurate than the measurements using a mass spectrometer.

Components of the mass spectrometer include:

Ion Chamber,

Velocity Selector,

Momentum Selector,

Ion Detector.

Measuring the mass
Once the ions have been acclerated to the same velocity in the velocity selector they enter the momentum selector. There is no electric field present but instead a magnetic field, the centripetal force on the ions is provided by the magnetic force,

BQv=mv(2)/r           Flux Density x Charge x velocity = mass x velocity squared / radius

The radius of the Ion beam's path is given by

r=mv/BQ

The ion beam consists of singly ionised particles of constant velocity travelling in a unifrom magnetic field. Thus radius is directly proportional to mass. This is an extremly effective way of distinguishing between different isotopes of an element - allowing the relative abundance of the isotopes to be found from the relative densities of the signals detected by the detector.